In the construction of integrated circuits, device geometries are constantly shrinking, resulting in an increase in parasitic capacitance between devices. Parasitic capacitance between metal interconnects on the same or adjacent layers in the circuit can result in crosstalk between the metal lines or interconnects and in a reduction of the response time of the device. Lowering the parasitic capacitance between metal interconnects separated by dielectric material can be accomplished by either increasing the thickness of the dielectric material or by lowering the dielectric constant of the dielectric material. Increasing the thickness of the dielectric materials, however, does not address parasitic capacitance within the same metallized layer or plane.
As a result, to reduce the parasitic capacitance between metal interconnects on the same or adjacent layers, one must change the material used between the metal lines or interconnects to a material having a lower dielectric constant than that of the materials currently used, i.e., silicon dioxide (SiO.sub.2), k.apprxeq.4.0.
Jeng et al. in "A Planarized Multilevel Interconnect Scheme with Embedded Low-Dielectric-Constant Polymers for Sub-Quarter-Micron Applications", published in the Journal of Vacuum and Technology in June 1995, describes the use of a low dielectric constant polymeric material, such as parylene, as a substitute for silicon dioxide (SiO.sub.2) between tightly spaced conductive lines or other strategically important areas of an integrated circuit structure. Parylene, a generic name for thermoplastic polymers and copolymers based on p-xylylene and substituted p-xylylene monomers, has been shown to possess suitable physical, chemical, electrical, and thermal properties for use in integrated circuits. Deposition of such polymers by vaporization and decomposition of a stable cyclic dimer, followed by deposition and polymerization of the resulting reactive monomer, is discussed by Ashok K. Sharma in "Parylene-C at Subambient Temperatures", published in the Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 26, at pages 2953-2971 (1988). Properties of such polymeric materials, including their low dielectric constants, are further discussed by R. Olson in "Xylylene Polymers", published in the Encyclopedia of Polymer Science and Engineering, Volume 17, Second Edition, at pages 990-1024 (1989).
Several parylene films have been developed for deposition within integrated circuits. Parylene-N is deposited from unsubstituted p-xylylene at substrate temperatures below about 70-90.degree. C. The parylene-N films typically do not adhere well to silicon oxide and other semiconductor surfaces. Furthermore, parylene-N films typically have poor thermal stability at temperatures above about 400.degree. C. Thermal stability of parylene films is improved by fluorinating or chlorinating the cyclic dimer of p-xylylene to make parylene-F films or parylene-C films. However, the substituted p-xylylene cyclic dimers are even more expensive than the unsubstituted cyclic dimer and are more difficult to process. In addition, the fluorine or chlorine within the films can corrode metal electrical interconnects when an electrical bias is applied.
The p-xylylene cyclic dimer typically used as the parylene precursor is formed as a solid that is separated from the reactants and other products during formation of the cyclic dimer. Although the cyclic dimer is very expensive, the cyclic dimer provides a controllable deposition process for making parylene films having low dielectric constants. The cyclic dimer is produced from p-xylene, or a derivative thereof.
P-xylylene copolymers having thermal stability above 400.degree. C. and dielectric constants as low as 2.1 are described in Taylor et al., Parylene Copolymers, Mat. Res. Soc. Symp., Proc. Vol. 476 (1997), pp. 197-205. P-xylylene is condensed on a substrate in the presence of a comonomer, such as tetravinyltetramethylcyclotetrasiloxane, and the resulting copolymer film is stated to have better thermal stability and lower dielectric constants than parylene-N films.
P-xylylene copolymer films having lower dielectric constants than about 2.1 would further improve on integrated circuit performance. However, useful comonomers for making the films have not been identified.